Method and apparatus for qualitative electromagnetic surveying



March 22, 1960 M. PURANEN 'ETAL 2,929,984

METHOD AND APPARATUS FOR QUALITATIVE ELECTROMAGNETIC suavzvmc Filed Jan. 12, 1956 3 Sheets-Sheet 1 FIGURE l FIGURE 2 MAMWH/RANEN, AAR/VO AssAR/QWA M0 llv/vo Rom/(A PATQVT Aas/vr Filed Jan.

M. PURANEN ET AL METHOD AND APPARATUS FOR QUALITATIVE ELECTROMAGNETIC SURVEYING 3 Sheets-Sheet 2 s RECEIVING FILTER PHASE cgI fl) METER AMPLIFIER REcoRoER TUNED a REFERENCE RECEIVING coI PHASE AMPLIFIER REcoRoER (m If= METER TUNED REFERENCE COILUZ) FIGURE 3 e REFERENCE COIL RECEIVING A PHASE sERvo 52''; DETIESTOR AMPLIFIER Iao SERVO PHASE NTROLL HIFTE TTEIIuAr PHASE oET gToR AMPLIFIER REcoRoER 90 PHASE SHIFTER FIGURE 4 March 22,1960 M. PURANEN ETAL METHOD AND APPARATUS FOR QUALITATIVE ELECTROMAGNETIC SURVEYING 3 Sheets-Sheet 3 Filed Jan. 12, 1956 METHOD AND APPARATUS FoR QUALITAHVE ELECTROMAGNETIC SURVEYING Maunu Puranen and Aaron Assar Kahma, Helsinki, Finland, and Vaino Ronka, Ajax, Ontario, Canada, ass'lgn'ors to Canadian Airborne Geophysics Limited, Toronto, Ontario, Canada Application .lanuary 12, 1956, Serial No. 558,804 10 Claims. (Cl. 324-6) This invention relates to a means and method of detesting and measuring the conductivity of bodies for geophysical and other purposes, and more particularly to such a means and method adapted to eliminate difl'lculties in conventional means and method which arise when the detector is moved continuously with respect to the bodies to be detected and at a considerable distance therefrom.

Electromagnetic prospecting is well known in the art. In this type of prospecting, a transmitting coil is supplied with an alternating electrical current, thus setting up an alternating magnetic field (usually termed the primary field). If this field comes into contact with a conductive body, it will cause alternating eddy currents to be induced in such body, which eddy currents set up their own alternating magnetic field (usually termed the secondary field). This secondary field may be detected by a receiving coil and the measure of this field will give an indication of the nature of the conductive body in question. This secondary field, because the conductive bodies possess both resistance and inductance properties, will be more or less out-of-phase with the primary field, that is the secondary field will have a quadrature component with respect to the primary field. H

The measure, of this quadrature component, as well as the amplitude of the secondary signal can be used to indicate the presence and relative conductivity of the conductive bodies producing the secondary field.

When the art of electromagnetic prospecting was in its infancy, all measurements were made from fixed positions. Lately, however, interest in airborne electromagnetic prospecting has become wide-spread. In such prospecting the transmitting coil and the alternating current generator have, because of the large size required for optimum results, been located in the prospecting aircraft,

while the receiving coil has been towed beneath the aircraft in a glider or bird, so as to bring the receiving coil as close as possible to the source of the relatively weak secondary field. With such 'an arrangement, the re ceiving coil is exposed, not only to the secondary field to be measured, but also to the relatively much stronger primary field from the aircraft.

7 2,929,984 Patented Mar. 22, 1960 2 elimination of. spurious data introduced by the relative motions of theaircraft and bird.

As has been pointed out, electromagnetic detection of conductive-bodies may be divided into two general classes.

(1) Amplitude measurement.-Measurement of themagnitude of the secondary field without regard to phase;

A number of factors influence the flight of an aircraft with the result that small variations of speed, attitude and altitude occur frequently. These changes influence the mutual spacing of aircraft, bomb and geologic stru'cture; the attitudes of aircraft and bomb are also affected. Since the primary field is relatively strong at the receiving coil, and since the strength of the field varies as the third power of the distance between the coils, a minor variation in the spacing between aircraft and bomb, 'can cause a diflerence in the amplitude of thepriniary signal which would in many cases mask entirely the efiect of the secondary signal which is relatively weak. Relative angular displacements between transmitting and receiving coils can also change the amplitude of the received signal. Therefore a major problem in (2) Phase measurement.---(a)- Measurement of in phase and quadrature components separately, and (*b) measurement of quadrature component alone- This may be done by measuring the phase angle between the primary and total fields (0 in Fig. 2) or by measuring the magnitude of the quadrature component.

Theoretically, quadrature component measurement has been found most effective for bodies, of higher re-- sistance since such bodies provide the greatest relative quadrature component, whereas prospectors are general" 1y interested in bodies of very low resistance. However, the phase measurement system has many practical advantages as will hereinafter be pointed out in detail,

which outweigh this consideration.

It is a principal object of this invention, therefore, to provide a means and method of electromagnetic detection which will eliminate the difficulties heretofore caused through variations in the relative position between transmitting and receiving coils.

It is another object of this invention toprovide means and method of electromagnetic surveying which willeliminate the possibilityof receiving spurious data'at the receiving coil through variations in the intensity of the primary field at the receiving coil.

It is still a further object of this invention to provide a phase measurement means and method whereby a better differential may be madebetween conductive bodies of high resistance, and conductive bodies of probable commercial interest, I 7

These and other advantageous objects will become apparent through a consideration of the following detailed description taken in conjunction with the attached draw ings in which: a

Figure 1 is a graph showing the variation in magnitude of in-phase' component (H .and quadrature components (H of the secondary field signal with respect to the frequency of the transmitted field and conductiv ity of the ore body. I

Figure 2 is a vector diagram illustrating the effect of the secondary field'on the signal received at the receiving" I ticul'ar set of conditions, varies with the frequency of the primary field. Although the phase of the secondary field reaches a limit of with respect to the primary fieldat zero frequency, in practice this is not readily measured, since the amplitude of the signal derived from the secondary field tends to zero at zero frequency. Figure 1 represents the response of in-phase and quadrature components at two frequencies .as a function of mineral body conductivity. In this graph, the combined amplitude and phase variations result in a peak of quadrature component response. If the response of only one frequency be considered, it is apparent that there exist two values of conductivity for each amplitude response exc'ep't'the-pealt value; When'a measurement underth'e for the phase angle mea'su-rer'ne nt" same conditions is made at another frequency, the ambiguity can be resolved by observing the ratio of the two responses.

1 This invention makesuse of this fact by providing two transmitting frequencies, and two receiving coils mounted with their axes horizontal and parallel and relatively close together compared with the separation between the transmitting and receiving coils, whereby the phase angle, 0, at each frequency may be detected and recorded and employed to give both an indication-of the presence of a conductive body and an indication of the properties (conductivity) of such body. The tworeceiving coils are mounted in a bomb-shaped enclosure (or bird) which is towed beneath the aircraft. a

It is an essential feature of this invention that transmitted fields of two different frequencies be employed. This may be done by using a single transmittingcoil in av doubly resonant circuit with two generators of the appropriate frequencies, or alternatively, two separate transmitting coilsmay be used, each supplied by a generator of the prescribed frequency. a

The frequencies involved in electromagnetic prospecting are not critical and are well known in the art. In general they will not be more than kilocycles. Ac-- cording to the method of this invention it is necessary to have a substantial separation between the two frequencies to produce a measurably different response in the ore body. In addition such a substantial preparation will permit isolation of the two frequencies and processing in separate channels. A suitable pair of frequencies would be 400 and 3000 cycles.

The transmitting coil, located in the aircraft, is placed horizontally, i.e. with its magnetic axis vertical.

Each receiving coil is usually, although not necessarily,- tuned to the related transmitted frequency and by means of suitable filters, etc. is connected in a circuit with suitable measuring and detecting means preferably located in the prospecting aircraft (see Figures 3 and 4) a In operation, each primary field intersects the receiving coils and induces a strong in-phase primary voltage in the appropriate receiving coil (H in vector diagram, Figure 2). In the presence of a conductive body, the

; ponent are directly proportional to eachother.

deviations of the receiving coils likely to be encountered in practice.

The quadrature component can also be simply computed from the phase angle 0 between the total field H; and the primary field H affecting the receiving coil. If phase angle 0 is small, as generally is the case in airborne surveys, the phase angle and the quadrature com y measuring the phase angle, we thus also are able to determine the quadrature component, and conversely. A simple way to measure the phase angle is illustrated by the functional block diagram in Figure 3. The phasemeter may be one of several electronic circuits familiar transmitted frequency.

t of a conductive body, and a comparison of the phase angle 0 at each frequency will give an indication as t -.in practice will be a resistor) and the output from this, detector is amplified and used to position an attenuator resulting secondary field induces a secondary voltage in the appropriate receiving coil (H in vector diagram, Figure 2). This secondary voltage is composed of an in-phase and a quadrature component H and H respectively. I

The in-phase component H is very small when compared with the primary component H so that the sum of H and H does not change appreciably when H; varies due to the presence of conductive bodies as'encountered in a'geophysical survey. H in practice does vary from zero to a value such as to give a phase angle of d and the total field, H has a phase angle 6 compared to the primary field. e a

It should be noted that if the primary component H is increased or decreased through a variation in the relat-ive position between transmitting coil and receiving coil, no false signal is given but rather thensensitivity of the system (i.e. phase angle 9) is merely increased or decreased slightly. If amplitude were measured, however, such a variation would introduce a false signal tending to confuse the observer. Such a variation in the relative positions of receiving coil and transmitting coil will frequently be encountered in airborne electromagnetic survey work Where variations in flight conditions will cause the towed bomb to occupy varying'positions and attitudes. 7 Since in this application the receiving coils have their 7 common axes approximately aligned with the primary field vector, the variation of the primary field component H with receiving coil attitude, obeys a cosine function, and'does not change appreciably for the small angular to those skilled in the field. The reference coil is a coil placed in vicinity of transmitting coiland adapted to pick up a signal from the primary field. Each coil is tuned to its operating frequency to attenuate the other To avoid any influence from van relative phase is indicated by the recorder.

such as that shownschematically in Figure 3 whereby the phase angle 0 may be measured and recorded for each transmitted frequency. The existence of a phase shift at either frequency is an. indication of'the presence the class or size of the material involved.

An alternative phase method is illustrated in Figure-,4. In this technique the in-phase component is reduced to a very small magnitude before extracting and measuring the quadrature component magnitude. This method imposes less stringent requirements on the phase detectors.

than if a single phase detector is used to measure the magnitude of the quadrature component directly.

The received signal and reference phase are derivedfrom the receiver coil in the bird and the reference coil,

adjacent to the primary coil respectively, as in the previous method. The in-phase reference voltage is injectedv into the phase detector (1) through impedance Z (which which controls the magnitude of inverted reference voltage injected through Z (which will also be a resistor in practice) into the input of phase detector (1). The: attenuator can be any convenient motor driven potentiom eter or electronic attenuator both of which are well known in servo equipment. This forms a negative feedback loop which tends to maintain the in-phase component at junction A at a minimum. The resultant signal at this point is almost entirely quadrature component if any secondary field is present (depending on the loop gain) audit is appliedto phase detector (2) which has a reference phase. in quadrature with the in-phase voltage. 'This phase detector acts as a second stage of selection for, the quadra- -ture component and the output is amplifiedand recorded.

can be of conventional No limiting is involved in this technique and the recorded signal is a measure of the quadrature component magnitude. A similar system can be employed for the second, frequency. In this method, as in the other, the circuits form according to the fun'ctionall descriptions specified.

,The operation of the reference to Figure'5. In this figure 1' is the primary field shown rows S.

Receiving coil 6 receives the second field of frequency: f while receiving coil 7 receives the secondary field f f and I; are separately recorded as shown n the drawing ation in amplitude of the received signal, the signal can be amplitude-limited before it reaches the phase meter and therefore only the entire system may be seen byi;

A.C. gen- I erator for frequency f and 2 the generator for These.

reference coils 8 and 9 detecting frequencies f, and f, respectively and feeding these reference signals for recordal of the phase shift of f and f secondary signals.

There has been described above several specific embodiments of the invention which forms the subject matter of this application. It is to be understood, however, that precise circuitry outlined and process means are to be construed as exemplary only. Such embodiments of the invention as fall within the scope and purview of the appended claims are to be considered as part of this invention.

What we claim as our invention is:

1. Apparatus for electromagnetic detection and analysis of conductive bodies which comprises two electric generators of difierent frequencies, means for radiating the output of said generators; said radiating means thus being operable to produce two separate primary alternating magnetic fields of said frequencies, thereby causing secondary fields to be generated by currents induced in said conductive bodies, two receiving coils, mounted with their axes parallel separate detecting means connected to each receiving coil, each said detecting means being operable to measure one of the amplitudes of the quadrature component of the secondary field and the phase angle between the primary field and the combined primary and secondary fields linking its coil.

2. Apparatus for airborne electromagnetic detection and analysis of conductive bodies which comprises two electric generators of different frequencies and at least one transmitting coil connected to said generators, all mounted in the prospecting aircraft, said transmitting coil thus being capable of producing two separate primary alternating magnetic fields of said frequencies, a bird towed beneath said prospecting aircraft, two receiving coils mounted in said bird with their axes parallel, and separate detecting means in circuit with each such receiving coil and operable to detect the magnitude of the quadrature component of the secondary field induced in said conductive bodies by one of the primary fields.

3. Apparatus for airborne electromagnetic detection and analysis of conductive bodies which comprises two electric generators of ditferent frequencies, at least one transmitting coil in circuit with said generators, all mounted in the prospecting aircraft, said transmitting coil thus being capable of producing a pair of primary alternating magnetic fields, a bird towed beneath said prospecting aircraft, two receiving coils mounted in said bird with their axes parallel, and separate detecting means in circuit with each such receiving coil and operable to measure the phase angle between one of the primary fields and the total field linking said coil.

4. Apparatus for airborne electromagnetic detection and analysis of conductive bodies which comprises two electric generators of different frequencies and a transmitting coil in a doubly resonant circuit with said generators, all mounted in the prospecting aircraft, said transmitting coil thus being capable of producing two separate primary alternating magnetic fields of said frequencies, a bird towed beneath said prospecting aircraft, two receiving coils mounted in said bird, each of said receiving coils being tuned to one of the generator frequencies, and separate detecting and recording means in circuit with each such receiving coil and operable to measure and record the magnitude of the quadrature component of the secondary field induced in said conductive bodies by one of the primary fields.

5. Apparatus for airborne electromagnetic detection and analysis of conductive bodies which comprises two electric generators of different frequencies, at least one transmitting coil, in circuit with both of said generators, a pair of reference coils tuned to difierent ones of said frequencies, all mounted in the prospecting aircraft, said transmitting coil thus being capable of producing a pair of primary alternating magnetic fields, a bird towed beneath said prospecting aircraft, two receiving coils mounted in said bird, each of said receiving coils being tuned to one of the generator frequencies, and separate detecting and recording means in circuit with each such receiving coil and each said reference coil and operable to measure the magnitude of the quadrature component of the secondary field induced in said conductive bodies by one of the primary fields.

6. A method for electromagnetic detection and analysis of conductive bodies which comprises simultaneously producing two separate and primary alternating magnetic fields of different frequencies, thereby causing secondary fields to be generated by currents induced in said conductive bodies, detecting the combined primary and sec ondary fields for each of said frequencies at a location spaced from said conductive bodies, and detecting one of the magnitude of the quadrature component of the secondary field and the phase angle between the primary field and the combined fields for each of said frequencies.

7. A method for electromagnetic detection and analysis of conductive bodies which comprises simultaneously producing two separate and primary alternating magnetic fields of different frequencies, thereby causing secondary fields to be generated by currents induced in said conductive bodies, detecting the combined primary and secondary fields for each of said frequencies at a location spaced from said conductive bodies, and detecting and recording one of the magnitude of the quadrature component of the secondary field and the phase angle between the primary field and the combined fields for each of said frequencies.

8. A method for electromagnetic detection and analysis of conductive bodies which comprises simultaneously producing two separate and primary alternating magnetic fields of difierent frequencies, thereby causing secondary fields to be generated by currents induced in said conductive bodies, detecting the combined primary and sec ondary fields for each of said frequencies at a location spaced from said conductive bodies, and detecting and recording separately the magnitude of the quadrature component of each secondary field.

9. A method for electromagnetic detection and analysis of conductive bodies which comprises simultaneously producing two separate and primary alternating magnetic fields of different frequencies, thereby causing secondary fields to be generated by currents induced in said conductive bodies, detecting the combined primary and secondary fields for each of said frequencies at a location spaced from said conductive bodies, and detecting and recording separately the phase angle between the primary field and the combined fields for each of said frequencies.

10. The apparatus of claim 1 in which said transmitting means includes a coil in a doubly resonant circuit with said generators, said receiving coils being tuned to different ones of the generator frequencies, and including separate recording means for each detecting means to record the output thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,623,924 Cartier Dec. 30, 1952 2,642,477 Puranen June 16, 1953 2,794,949 Hedstrom June 4. 1957 

